Jacketed reactor fuel element



United States Patent f JACKETED REACTOR FUEL ELEMENT Karl F. Smith,Downers Grove, and Ray J. Van Thyne,

Oak Lawn, 111., assignors to the United States of America as representedby the United States Atomic Energy Commission No Drawing. ApplicationNovember 8, 1957 Serial No. 695,470

3 Claims. 01. 204-1932 This invention deals with new ternary alloys andin particular with vanadium-base alloys and fuel elements jacketed withthese alloys.

Uranium fuel elements for sodium-cooled fast neutronic reactors, such asthey are described, for instance, in copending application, Serial No.437,017, filed by Walter H. Zinn on June 15, 1954, now Patent No.2,841,545, granted July 1, 1958, grow under the effect of radiationwhich causes jamming of the fuel elements; operation of such reactors istherefore hazardous. It has been tried to enclose the fuel elements ofuranium metal, which is to include elemental uranium anduranium-containing alloys, with a jacket or can that has a high strengthso that the fuel elements are restrained from expanding.

Zirconium and stainless steel have been used for this purpose, and someimprovement was obtained with the jackets made of these metals. However,there are certain disadvantages in connection with the jackets usedheretofore. For instance, the iron of the stainless steel forms alow-melting (about 725 C.) eutectic with the uranium metal of the corewhich means destruction of the fuel element. Zirconium and the zirconiumalloys, primarily the Zirconium alloy containing from'l to 2.5 percenttin and up to 0.2 percent of iron, chromium, and nickel, do not showthis drawback, but they are not strong enough at elevated temperaturesto prevent the fuel element from growing. Other metals tested did notshow a suificiently good heat conductivity which is a vital factor forsatisfactory operativeness of the reactor. Of the two metals usedheretofore stainless steel was the preferred material for uranium fuelelement jackets.

It is an object of this invention to provide an alloy suitable forjacketing uranium fuel elements which does not have the disadvantagesset forth above.

It is also an object of this invention to provide an alloy for jacketinguranium fuel elements which is superior to stainless steel.

It is an object of this invention to provide an alloy for jacketinguranium fuel elements which does not form a comparatively low-meltingeutectic with uranium.

It is another object of this invention to provide an alloy for jacketinguranium fuel elements which has a sufficiently high mechanical strengthand good creep resistance at elevated temperatures so that the uraniumcore is retrained from growing.

It is still another object of this invention to provide an alloy forjacketing uranium fuel elements which has a good thermal conductivity.

It is furthermore an object of this invention to provide an alloy forjacketing uranium fuel elements which has a low neutron-capture crosssection.

It is another object of this invention to provide an alloy for jacketinguranium fuel elements which has good corrosion resistance to practicallyoxygen-free sodium.

Finally, it is also an object of this invention to provide an alloy forjacketing uranium fuel elements that can be fabricated and welded.

It was found that ternary vanadium alloys containing from 2.5 to 15percent by weight of titanium and from 0.5 to 10 percent by weight ofniobium have the characteristics required and set forth above. Thealloys containing about 10 percent of titanium and from 1 to 3 percentof niobium were the preferred compositions, and the alloy containing 10percent of titanium and 3 percent of niobium yielded the mostsatisfactory results.

Vanadium forms a eutectic with uranium which melts at 1040 C. Whenimmersed in sodium that contains some oxygen (the sodium in fastreactors usually has a negligibly low oxygen content, though) vanadiumforms lower comparatively stable oxides, such as V0 and V 0 which form abarrier film on the surface and protect the vanadium or alloy againstfurther corrosion at elevated temperatures.

Nonalloyed vanadium metal was first examined for the prime purpose ofthis invention, but it was found not to be superior, as to mechanicalstrength, to stainless steel 347 (17 to 19 percent of chromium, 9 to 12percent of nickel, up to 0.08 percent of carbon and niobium in aquantity tenfold of that of carbon), the steel heretofore preferred forjacketing uranium fuel elements for fast neutronic reactors.

The alloys of this invention can be prepared by any method known tothose skilled in the art. The inventors preferred melting in an arcfurnace. were heated to about 1350 C. and pressand hammerforged to/2-inchand/or Ai-inch-diameter bars. The bars were then ground to sizeon a belt centerless grinder that had a belt 4 inches x 54 inches and agrit size of 36; the grinding material was aluminum oxide.

All bars were tested as to tensile strength at room temperature and at650 C. at a crosshead speed of 0.05"/ min. after they had been annealedat 650 C. for tWo days and water-quenched. Similar experiments werecarried out at 800 C., the annealing conditions in that instance havingbeen 800 C. for 24 hours followed by waterquenching. The results ofthese tests together with the values obtained for yield strength,elongation and reduction of area are combined in the table below, andthe corresponding data for stainless steel and zirconium are added forthe sake of comparison. (Whenever heating of the alloys was carried out,it was done in an inert atmosphere of helium or argon gas.)

Tensile Yield Elonga- Reduc- Alloy Strength, Strength, tion, tion of p.s. i. p. s. i. Percent Area,

Percent;

V-10 Ti1 Nb 84, 600 63,200 11 20 V10 Ti3 Nb 97, 600 64, 500 22 38 V10Ti1 Nb 80, 400 58, 000 15 41 V-1O Ti3 Nb.-. 86,000 64, 000 21 48Stainless Steel 347. 51,200 41, 000 46 71 Zirconium 15, 200 5, 000 30 60Zr-alloy 1.4% Sn, 0.2 Fe, Cr,

V1O Ti1 Nb 55, 200 41, 600 13 33 Stainless Steel 347 23, 000

1 At 480 C. 2 N of; determined.

It is obvious from this table that tensile and yield strengths of thevanadium-titanium-niobium alloys are much superior to those of stainlesssteel and zirconium Patented Dec. 9, 1958 The ingots obtained metals atelevated temperature and that the ductility of these-new alloys islower, as is obvious from the data for elongation and reduction of area;however, the duetility is still su flicient to make the alloys suitableand workable for the purpose of this invention.

The vanadiumpercent T i-l percent Nb ternary alloy was also examined asto stress rupture strength by applying different loads and determiningthe number of hours that were required to cause rupture. From a loadtimecurve obtained from the results of these rupture tests the strengthbringing about exactly a 100-hour life was determined. It was found thatat 650 C. the alloy withstood a load of 62,000 lbs/sq. in. for 100 hourswhich compares with a strength of 27,000 p. s. i. for stainless steel347 at the same temperature. At 800 C. the stress rupture strength, for100 hours, was 22,000 p. s. i., which compares with a stress rupturestrength of 9000 p. s. i. for 100 hours with stainless steel 347 at 816C.

The alloys were furthermore tested for corrosion resistance in sodiumthat had a low oxygen content, such as it is present in the sodiumcoolant of fast neutronic reactors. (The oxygen content in fast reactorshas to be low to minimize transport corrosion" or in other words toprevent oxidation and pickup of the oxide formed from hot surfaces anddeposition on cold surfaces.) The oxygen corresponded to a content ofless than 0.001 percent by weight of Na O. For the corrosion test, thealloys were immersed in the sodium at 700 C. for twelve days, and theweight gain, which is an indication of the degree of corrosion, wasdetermined in each case. For the 10 percent titanium-1 percent niobiumand the 10 percent titanium-3 percent niobium vanadium-base alloys theweights increased by 0.17 and 01-19 percent, respectively.

Both alloys, that containing 1 percent and that containing 3 percent ofniobium, in addition to 10 percent of titanium, were found to besuperior to zirconium as to heat conductivity and at least as good asstainless steel 347. The alloys were welded in an inert atmosphere, e.g. of helium, using a tungsten electrode and an additional supply ofshielding gas to prevent contamination. The preferred conditions were awelding rate of 14"/ min., an electric current of amps. at 20 volts, anda total gas flow of 35 ft. /hr.; however, other conditions are alsosuitable.

The 10 percent Ti-l percent 'Nb ternary vanadiumbase alloy wascold-rolled (at room temperature, about 25 C.) satisfactorily'to 95percent of reduction in thickness whereby a 2.5-mil thick sheet wasobtained.

While the invention has been described primarily in connection withjackets for fuel elements, it is understood that the alloys can be usedfor any purpose where one or several of the properties of the alloys ofthis invention are of importance, for instance, they can be used asconstruction material of aircraft engines.

It is also understood thatthe invention is susceptible to variousmodifications and changes, and that it is to be limited only by thescope of the appended claims.

What is claimed is:

1. A fuel element for fast neutronic reactors, comprising a core ofuranium metal-containing material and a jacket around said core, saidjacket consisting of from 2.5 to 15 percent by weight of titanium, from1 to 5 percent of niobium and from to 96.5 percent of vanadium.

2. The fuel element of claim 1 in which the titanium content is 10percent and the niobium content is 1 percent.

3. The fuel element of claim 1 in which the titanium content is 10percent and the niobium content is 3 percent.

References Cited in the file of this patent .UNITED STATES PATENTS1,727,180 Saklatwalla Sept. 3, 1929 2,798,848 Kingdon July 9, 19572,799,642 Hurwitz et al. -July 16, 1957 2,805,153 Rostoker Sept. 3, 1957V

1. A FUEL ELEMENT FOR FAST NEUTRONIC REACTORS, COMPRISING A CORE OFURANIUM METAL-CONTAINING MATERIAL AND A JACKET AROUND SAID CORE, SAIDJACKET CONSISTING OF FROM 2.5 TO 15 PERCENT BY WEIGHT OF TITANIUM, FROM1 TO 5 PERCENT OF NIOBIUM AND FROM 80 TO 96.5 PERCENT OF VANADIUM.